Exhaust gas purification catalysts obtained by coating noble metals such as platinum, rhodium and palladium onto carriers are used for the purpose of efficiently purifying noxious gas components, HC, CO, NOx and the like emitted from internal combustion engines. Among the noble metals used for NOx purification by reduction reaction, rhodium exhibits particularly superior heat resistance compared to platinum or palladium. It is therefore particularly suited for automobile exhaust gas purification reactions. On the other hand, since rhodium is quite expensive compared to other noble metals, its use in catalysts is usually preferred to be minimal, and for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-282692 states that the loading weight of rhodium is preferably 0.1-2 g for 1 L of catalyst.
Noble metal solutions used in the prior art have had problems of low efficiency of loading with coating layers. Thus, since it has been difficult to load noble metals onto carriers in the desired amount by single dip treatment using conventional noble metal-containing solutions that have low loading efficiency, it has been necessary to carry out multiple dip treatments. Japanese Unexamined Patent Publication (Kokai) No. 2005-314739 attempts to solve this problem by loading the noble metal on the carrier at high density. However, while this publication mentions a palladium-containing solution (Example 1) and a platinum-containing solution (Example 2), there is no concrete disclosure of a solution for high-density loading of rhodium onto the carrier, and even the amounts of organic bases such as amines are not specified.
For efficient use of rhodium, it is known that the rhodium should be loaded preferentially so as to be concentrated near the surface of the wash coat layer for greater contact efficiency with the exhaust gas, and for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-301000 discloses a technique for loading noble metals such as rhodium on the surface of coating layers. However, the loading method used is laborious, and the advantage of such higher-density loading is not fully realized.